There is known a research report on a method for accurately measuring an ion beam current value without cutting off ion beams (refer to non-patent document 1). This method uses a sensor called SQUID that employs a Josephson junction device as a highly sensitive magnetic field sensor to detect a magnetic field generated by a beam current and measure the resulting beam current value. SQUID has a superconducting ring structure where two Josephson junctions are arranged in parallel and measures the magnetic flux that passes through the superconducting ring on the scale of the magnetic flux quantum (2.07×10−15 Wb).
In the above document, SQUID uses a low-temperature superconductor operating at a liquid helium temperature or below. The main section of the beam current measuring apparatus comprises: a detection part for detecting a magnetic field corresponding to a beam current; a magnetic flux transmission part for transmitting magnetic flux to a measuring part; a measuring part including a superconducting device and a feedback coil supplying a feedback current to cancel a change in the magnetic flux passing through the superconducting device; and a magnetism shielding part having a gap composed of a superconductor magnetically shielding the detection part and the measuring part from external space including ion beam flowing space.
The detection part that is a coil including a superconducting wire wound around a core of a soft magnetic material collects, by using the core of a soft magnetic material, a magnetic field generated by a beam current and induces a superconducting current on the coil.
The superconducting current induced on the coil is transmitted to a coil arranged adjacently to the SQUID. The superconducting current flowing through the coil changes as the beam current changes, thus causing a change in the magnetic flux amount passing through the SQUID. The detection part applies a feedback current to a feedback coil to cancel the change in the magnetic flux amount passing through the SQUID. The feedback current is proportional to a change in the beam current value. It is thus possible to determine the amount of change in the beam current value by measuring the feedback current.
Recently, there has been a research on a method for measuring a beam current value using a high-temperature superconductor (refer to non-patent document 2). According to the method disclosed in this document, a cylinder whose surface is coated with a high-temperature superconductor is used as a detection part, with only a portion of the outer peripheral surface of the cylinder including a bridge part of the high-temperature superconductor. A beam current that passes through the center of the cylinder induces a surface shielding current on the surface of the cylinder. The surface shielding current is concentrated on the bridge part. Magnetic flux generated by the concentrated surface shielding current is thus measured using the SQUID. The SQUID employed in this method uses a high-temperature superconductor and operates at a liquid nitrogen temperature or above.
Beam current measuring apparatus using the low-temperature superconducting SQUID can measure a beam current with a noise width of several nanoamperes.
Beam current measuring apparatus using the high-temperature superconducting SQUID is advantageous in that it operates using liquid nitrogen or a chiller although its noise width is as wide as several microamperes (refer to non-patent document 3). This apparatus shows a large drift of the zero point and measures only a beam current equivalent to 10 μA or above in actual measurements of several tens of seconds or above.
Another nondestructive measuring method is a direct-current transformer. This approach, however, shows a noise width equivalent to several microamperes. Considering the drift of the zero point, measurement of a beam current below 10 μA is practically difficult.
[Non-patent document 1] “Super Conducting Quantum Interference Devices and Their Applications” (Walter de Gruyter, 1977) p. 311, IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-21, BO. 2, MARCH 1985, Proc. 5th European Particle Accelerator Conf., Sitges, 1996 (Institute of Physics, 1997) p. 1627, Journal of the Physical Society of Japan Vol. 54, No. 1, 1999.
[Non-patent document 2] IEEE TRANSACTION ON APPLIED SUPERCONDUCTIVITY, VOL. 11, NO. 1, MARCH 2001 P.635
[Non-patent document 3] IEEE TRANSACTION ON APPLIED SUPERCONDUCTIVITY, VOL. 11, NO. 1, MARCH 2001 P.635